The invention concerns a self-cutting fast construction screw to fasten fiber plates to a metal substructure, with a screw shaft carrying at least one turn of a thread and passing over into a conical drilling point, and with two cutting edges located near the end of the shaft and capable of breaking off upon impact on the metal substructure.
To fasten mineral fiber plates and the like to a metal substructure consisting for example of sheet metal supports, customarily self-tapping construction screws are used (DE-OS No. 25 39 365), in which the core of the screw carries a two-turn (or double helix) thread, one turn of which extends to the conical point of the screw. After the penetration of the screw point into the metal substructure, this thread begins to cut a thread.
The screw threads in the mineral fiber plate to be fastened are also formed by the point of the screw, whereby the material is not removed from the bore, but merely displaced laterally. However, for high strength fiber plates solidly pressed of a fibrous material the use of these known construction screws causes problems, as the material to be displaced offers great resistance to the the screw, so that a relatively high axial force must be applied to make the penetration of the fast construction screw into the fiber plate possible. The threads provided for fastening in the metal substructure must pass through the fiber material, so that a finished thread is shaped into the compressed fiber material preventing the establishment of a solid joint between the steel substructure and the fiber material, since the fiber material is already compressed laterally, an appreciable frictional force is generated so that a very high torque must be applied to rotate the construction screw into the material.
In the case of construction screws of the aforementioned type, intended for the fastening of wood panels to a steel substructure, it is known to provide two opposing cutting edges on the lower end of the screw shaft, both of which cut into the wood to the outer diameter of the thread. The cutting edges produce a bore hole in the wood, through which the threads may pass without appreciable friction with the wood. The edges break off upon their impact on the metal. For the mounting of fiber plates made of high strength fibrous materials, however, these known construction screws are not suitable, because here again the drill point would have to produce the entire bore hole to the core diameter of the screw shaft.
It is therefore the object of the invention to develop a fast construction screw of the aforedescribed type that would be suitable for use with fiber plates made of high strength fibrous materials.